The present invention relates to a circuit for restoring the DC level of an electric signal and particularly to an improvement in a DC restoration circuit for use in the video processing circuit of a TV receiver.
In a TV receiver, the pedestal level of a video signal is clamped at a prescribed potential by a DC restoration circuit. Conventionally, a predetermined fixed DC level is applied to the clamping potential. However, in certain cases, it is preferable to somewhat change the value of the clamping potential in accordance with the contents of video signal. To achieve such clamping potential changing, a specific circuit, specifically designed to adjust the clamping potential, is required.
When this specific circuit is combined with a DC restoration circuit, it is possible to optionally change the rate of DC restoration. However, according to this circuit configuration, if an input coupling capacitor for receiving a video signal input is also employed as a clamping capacitor, an amplified output signal of the DC restoration circuit could be positively fed back to the input side thereof. This positive feedback often causes disadvantages such as variations in the brightness of the video picture, unfavorable oscillations in the circuit, etc. Further, since the circuit impedance at the input coupling capacitor is generally designed to be high, the circuit is liable to be affected by external disturbances such as noises.